Existing pin diodes and avalanche photodiodes (APDs) are sensitive for a particular wavelength range, depending on their specific absorption bands. For example Si (silicon) APDs are sensitive in the range of 450-1000 nm, Ge (germanium) APDs spectral response range covers 780-1700 nm, whereas InGaAs (indium gallium arsenide) APDs are responsive at 800-1650 nm.
Examples of APD application include receivers in optical fiber communications, range finding, imaging, high-speed laser scanners, laser microscopy, and optical time domain reflectometry.
InGaAs APDs are more expensive than Ge APDs, but have better noise performance. Due to its narrow energy bandgap, Ge exhibits high dark current at room temperature, and high multiplication noise due to very similar electron and hole ionization coefficients.
When operated in the Geiger-mode (reverse bias above the breakdown voltage, Vbr), APDs can be used as SPADs (single-photon avalanche diodes). When an electron-hole pair is generated by the absorption of a single photon in a SPAD, it can trigger a strong avalanche current. Geiger-mode performance is primarily characterized by the single-photon quantum efficiency (SPQE). The SPQE is defined as the probability that a photon triggers an avalanche breakdown and no dark carrier triggers a breakdown (dark-count) given that an optical pulse is present and at least one photon enters on the SPAD. SPQE is defined as
      S    ⁢                  ⁢    P    ⁢                  ⁢    Q    ⁢                  ⁢    E    =                    (                  1          -                      P            d                          )            ⁢              P        opt                    p      o      where Pd is the dark-count probability, Popt is the probability that at least one photon triggers an avalanche breakdown, and Po is the probability that at least one photon enters on the SPAD during the detection time.Pd=1−e−QaNd where Qa is the probability that a dark carrier triggers an avalanche breakdown and Nd is the average number of dark carriers generated in the multiplication region during the detection time.Popt=1−e−ηPaNo where No is the average number of photons per pulse, Pa is the probability of an avalanche breakdown caused by a photo-carrier that is injected into the multiplication region, and η is the quantum efficiency of the SPAD, which is the probability that an impinging photon is absorbed and converted into an electron-hole pair. Also, the probability that at least one photon is present in the optical pulse is given bypo=1−e−No 
From the above equations, it is therefore necessary for SPADs to have low dark current characteristics to minimize “dark count”.
Applications that use SPADs can be grouped into 2 general categories depending on the wavelengths to be detected. Si-based SPADs are typically used for visible spectral range up to 1-μm wavelength, while InGaAs/InP SPADs are used for wavelengths from 1-μm to about 1.8 μm. Si and InGaAs/InP based SPADs typically suffer from high dark current.
A need therefore exists to provide an avalanche photodiode that can address high dark current found in existing APDs and SPADs.